1. Field of the Invention
The present invention relates to a vehicle suspension system for a steerable drive wheel and, more particularly to a vehicle suspension system which makes it possible to set up an imaginary kingpin axis to optimize the characteristics of the geometry with respect to the vehicle steering, improves handling safety by setting up an imaginary link and minimizing the variation ratio of the height of a roll center, enhances free layout degree with respect to a change in the angle of camber and tread, and improves driving safety by lowering a mounting point of the shock absorber and a hood line.
2. Description of Related Art
The primary functions of a suspension system are to (1) provide vertical compliance so wheels can follow an uneven road, thereby isolating a vehicle body from roughness in the road, (2) maintain the wheels in a proper steer and camber attitudes to the road surface, (3) react to a control force produced by tires-longitudinal acceleration and braking forces, lateral force, and braking and driving torques, (4) resist roll of the vehicle body, and (5) keep the tires in contact with the road with minimal load variations.
The common designs for the steerable wheel of the vehicle are of Wishbone and MacPherson types. FIGS. 5A and 5B illustrate a conventional MacPherson type suspension system which comprises a wheel carrier 100, a strut assembly 104 having a shock absorber 102 and 103, a lower arm 106, and a ball joint 108. The MacPherson strut provides major advantages in package space for a space for a transverse engine.
When the offset A becomes small, the knuckle loads and kingpin loads are reduced and the tendency of toe-out is reduced during driving. In order to make the offset A smaller or negative (-), a supporting point 110 of the strut assembly 104 should be moved to the engine room side or the ball joint 108 should be moved to the outside of the wheel. However, in case of the former, the effective volume of the engine room is reduced and the kingpin angle .alpha. becomes too large, which adversely affects cornering performance. In case of the latter, it is difficult to move the ball joint toward the wheel, because of the interference with the brake disc.
FIGS. 6A and 6B illustrate a conventional Wishbone type suspension system 128 which comprises upper and lower arms 120 and 122, a steering knuckle 124, a shock absorber 126, and ball joints 130 and 132 for connecting the upper and lower arms to the steering knuckle 124, respectively. In the Wishbone type suspension system, the problem of making the offset A smaller or negative (-) is similar to the MacPherson type suspension system.
As described above, in the conventional MacPherson and Wishbone type suspension systems, a free layout degree for establishing the kingpin angle .alpha. is limited within a small range and there is a limit to improve the functions of suspension systems. Also, since the bounce and rebound of the wheel is dependent on a short control link such that the variation rate of the height of the roll center becomes large, the driving stability is relatively lacking. Further, since the number of the links of the suspension systems are small, the impacts applied to the vehicle body cannot be spread over a wide area and the ability of the suspension system to absorb impacts is not good, which causes the ride comfort and impact safety to be poor.